This invention relates generally to integrated circuits and manufacturing methods and more particularly to programmable read only memory (PROM) integrated circuits which include fusible links as programmable memory elements thereof.
As is known in the art, programmable read only memory integrated circuits have a wide range of applications in digital computation and processing systems. As is also known in the art, such PROM circuits are typically formed as a single semiconductor integrated circuit chip. In bipolar PROM circuits, a matrix of rows and columns of conductors have programmable memory elements connected between unique row and column conductor combinations. Each one of the programmable memory elements typically includes a diode and a serially connected fusible link. During programming, selected ones of the fusible links are blown, creating an open circuit between the previously electrically connected row conductor and column conductor connected to such blown fusible link. The resulting pattern of blown and unblown fusible links represents the data stored in the PROM. More particularly, a blown fusible link at a "location" defined by the unique row conductor-column conductor combination previously connected to such blown fusible link may represent a logical 0 signal stored at such location; whereas an unblown fusible link at a second address defined by a different row conductor-column conductor combination may represent a logical 1 signal stored at such second address.
As is also known in the art, one method used to form each one of the memory elements is to provide a semiconductor and deposit an insulating material such as silicon dioxide over a surface of the semiconductor. A window is formed in a region of the semiconductor where the diode portion of the memory element is to be formed to expose the underlying surface portion of the semiconductor. A Schottky diode is then formed in the semiconductor. A layer of fusible material, such as a nickel-chromium compound, commonly referred to as nichrome, is deposited over the surface of the insulating material. The fusible material is then selectively masked and etched so that a fusible link is formed on a portion of the insulating material adjacent the window. Next, a first metallization layer is deposited over the fusible link and over the exposed insulating material and through the window onto the Schottky diode region. The metallization layer is then patterned into a column conductor in contact with a first end of the fusible link and a connector to connect a second end of the fusible link to the diode. In order to provide a second layer of metallization for say the row conductors of the PROM circuit, a second layer of metallization is generally formed over the structure including a portion of the fusible link between the ends thereof as well as a portion of the metallization layer of the row conductors. One technique used to provide the second layer of metallization has been to chemically vapor deposit silicon dioxide over the surface of the structure. The deposition is generally done in atmospheric pressure so that it provides a good moisture barrier for the nichrome fusible link. However, with such deposition technique the chemically vapor deposited silicon dioxide forms cusps around the edges of the conductors making such insulating material difficult to metallize over for the second metallization layer. One technique used to reduce the effect of the cusps has been to form the first metallization layer with sloped or bevelled edges using conventional sloped etched techniques. One technique used uses a mask which allows some of the etchant to flow under the edge portions of the mask thereby resulting in the sloped etch. Such technique therefore requires that the mask be slightly larger than the desired metallization width thereby reducing the packing density of the device. While the use of an insulating layer of silicon nitride deposited directly on the nichrome has been suggested, such direct deposition changes the resistive value of the nichrome fusible link. Further, when such silicon nitride layer is deposited directly on the nichrome fusible link and such link is blown it has been found that the silicon nitride ruptures thereby breaking down the desired dielectric insulation.